Hi @Barry thanks for the reply. I upgraded to Mojave about six weeks ago. I’ve used SketchUp Pro 2016 for many many hours since that time, both actively sitting in front of it doing modeling operations, and letting it sit idle overnight. I almost never quit out of SketchUp, I normally have a session last for a month or more. This string-NULL-not-valid issue only started mysteriously a couple of days ago.
I understand that 2016 + Mojave is not a supported configuration (sob!). I would rather not use any later versions because I really dislike their support for high-DPI displays (at least on the Mac). The model’s edges and guides drawn in the viewport are thin single-pixel width in 2016, which allows me to see wonderful detail (screw threads, fillets, etc.) even with hidden geometry enabled. 2017 and later draw edges and guides with relatively thick lines (on all recent releases of Mac OS, not just Mojave), which obscures details until zooming in further than I would prefer. See the following post for a recent discussion of that topic: